The present invention is directed to apparatus and methods for pulping or defibering paper pulp of high consistency, e.g. between 12% and 25% solids, and more particularly, to pulping apparatus in which a rotor circulates pulp stock within a tub.
Pulping apparatus for defibering waste paper and the like typically include a vat or tub within which is mounted at least one rotor or impeller for circulating the paper stock to be defibered. The rotors presently in use effect defibering of pulp stock by creating mechanical shear and/or hydraulic shear conditions which act on the pulp to reduce its particle size to predetermined maximum dimensions. Mechanical shear is achieved by the interaction of a moving surface, typically on a rotor, with a stationary surface, typically the bottom wall or bed plate above which the rotor is mounted. Hydraulic shear occurs when pulp fibers contact other pulp fibers in the stock as a result of the turbulence or flow pattern generated by the rotor within the tub.
Most pulping devices currently in use are capable of defibering pulp stock up to a maximum consistency of approximately 5% to 8% solids, such as the pulping apparatus disclosed in Couture U.S. Pat. No. 4,109,872, commonly assigned. That patent discloses pulping apparatus having a rotor with a relatively flat body and a plurality of vanes extending generally radially outwardly from the body. Rotation of the rotor creates both hydraulic and mechanical shear in the vicinity of the rotor vanes. However, such pulping devices are incapable of pulping paper stock having a consistency higher than about 10% solids, because stock of that high consistency will not circulate in the tub in response to the rotation of such a rotor.
It is desirable to pulp paper stock having higher consistencies of, for example, between 12% and 25% solids. Not only can a given tub hold a greater volume of pulp for a given batch to be processed, but the hydraulic shear created in the higher density pulp, if it can be circulated properly, defibers the stock at a faster rate than for low density pulp. However, pulping devices of the type previously described are incapable of pulping paper stock at these higher consistencies. Due to the high viscosity of high consistency pulp stock, the rotation of the rotor would displace pulp stock outwardly from the vicinity of the vanes, but the stock immediately above the rotor would not flow downwardly to fill the void, creating a condition of rotor cavitation. With such cavitation conditions, the pulp stock is not circulated, and the desired hydraulic and mechanical shear effects cannot occur.
Rotors have been designed which include means for urging the stock from the center of the tub downwardly into contact with pumping members. For example, Wallen U.S. Pat. No. 3,035,781, discloses a rotor comprising a body having a hyperbolic contour which supports a pair of flat screw flights. Each screw flight is helically shaped and includes a leading edge which is oriented substantially perpendicular to the axis of rotation of the rotor, and a trailing edge which is oriented substantially parallel to the rotational axis. When the rotor is rotated in the presence of pulp stock, the screw flights urge the pulp stock downwardly and cooperate with the hyperbolic body to pump the stock radially outwardly, but this patent does not disclose the consistency of any stock of which it had been used.
A disadvantage with such a rotor is that the screw flight design does not create mechanical shear conditions which would accelerate the defibering of the pulp stock, since the trailing portions of the screw flights do not interact with a stationary object. Furthermore, a substantial portion of the screw flight is inclined relative to the axis of rotation, and as a result would tend to urge pulp stock outwardly from the axis before the pulp stock reached the trailing pumping portions of the flights. Since this outwardly flowing stock would not contact the pumping portions, the hydraulic shear effect upon it would be reduced.
Accordingly, there is a need for a high consistency pulping apparatus which includes a rotor capable of generating both hydraulic and mechanical shear forces. There is also a need for a high consistency pulping device having a rotor which positively guides the pulp stock downwardly to engage the entire working face of the pumping means to maximize hydraulic shear forces.